1. Field of the Invention
The present invention relates to a multilayered tubular packaging casing for pasty materials, in particular a synthetic sausage casing, based on polyamide.
2. Description of the Prior Art
Polyamide based casings have gained increased importance as foodstuff packaging materials and, in particular, as synthetic sausage casings. Quite a number of different types of sausage meats require casing materials that exhibit low permeability to water vapor in order to ensure that the weight loss of the sausage as a result of water loss during storage is kept to a minimum. This will also ensure that the sausage casing surrounds the sausage meat in a tight, crease-free manner, even after relatively long periods of storage without formation of hollow spaces between the inner wall of the casing and the sausage meat where jelly pockets can form. For this purpose, it is known to combine a polyamide layer with a polymer layer whose permeability to water vapor is lower than that of the polyamide layer. There are, however, certain types of food with which even the reduced permeability of these composite films is too high to be suitable for use. For example, sausage meat of the liverwurst type discolors when it is exposed to atmospheric oxygen, and therefore casings of the type described above are not suited for these types of sausage meat. To reduce the permeability of oxygen through such casings, the polyamide film additionally requires an oxygen barrier layer.
In EP-A-0,305,959, a multilayered polyamide film is disclosed, which may, for example, be used in the form of a heat-sealable bag for packaging meat and poultry. The film has a barrier layer against oxygen and comprises an amorphous, partially aromatic copolyamide (PA 6I/6T) having units of hexamethylene diamine, terephthalic acid and isophthalic acid. This copolyamide layer constitutes the core layer of the film and is sandwiched between two polyolefin layers that form a barrier against water vapor, whereby an inner polyamide layer has the purpose of preventing moisture of the packaged food from penetrating into the copolyamide core layer. It is, however, not advantageous to use polyolefin layers as inner layers of sausage casings because they do not exhibit adequate adhesion with sausage meats and thus favor the formation of jelly pockets.
EP-A-0,127,296 (corresponding to U.S. Pat. No. 4,606,922) attempts to overcome this problem by providing an ionomer for the inner surface of a multilayered polyamide film and by treating the inner surface with ionizing radiation.
In accordance with DE-A-38 16 942 (corresponding to U.S. Pat. No. 4,888,223) the formation of creases on a multilayered polyamide casing can be avoided by subjecting the inner layer comprising a polyolefin resin to a corona discharge treatment and, optionally, by dusting the inner layer using a starch powder. The proposed solutions have the disadvantage that the inner surface must be subjected to an additional pretreatment in order to prevent the formation of creases and jelly pockets between the inner surface of the casing and the sausage meat composition.